Field
The disclosed concept relates generally to circuit breakers, and more particularly, to circuit breakers including a frame and a trip unit. The disclosed concept also relates to methods of providing a current rating associated with a circuit breaker frame to a trip unit.
Background Information
Circuit interrupters, such as for example and without limitation, circuit breakers, are typically used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Circuit breakers typically include separable contacts. The separable contacts may be operated either manually by way of an operator handle or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open the separable contacts, and a trip mechanism, such as a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
Circuit breakers include a frame and a trip unit. The frame includes all the various components of the circuit breaker except for the trip unit. The trip unit may be interchangeably used with frames of the same or a similar design. A family of circuit breakers may be a group of circuit breakers that use the same or similar frames.
Circuit breakers have an associated rated current (In) which is also commonly referred to as an ampere rating or continuous current rating. The rated current is the maximum continuous current a circuit breaker is designed to carry without tripping. A rating plug is a self-contained portion of a circuit breaker that is interchangeable and replaceable in a circuit breaker trip unit by the user to set the rated current of the circuit breaker. A frame size (also referred to as frame rating) is a term applied to a group of circuit breakers of similar physical configuration (e.g., without limitation, a family of circuit breakers). The frame size refers to the largest current rating available in the group.
A rating plug associated with a current rating that is higher than the frame size of a circuit breaker should not be installed in that circuit breaker. For example, a rating plug associated with an 4000 A current rating should not be installed in a circuit breaker with an 800 A frame size. This creates a dangerous situation as the circuit breaker will not trip until a current of 4000 A while the components of the circuit breaker are only certified for safe operation up to a maximum continuous current of 800 A.
To address this concern, a mechanical interlock scheme between circuit breakers and rating plugs has been employed. In more detail, rating plugs and circuit breakers are designed such that a rating plug having a higher current rating than the frame size of a circuit breaker will not physically be able to be plugged into the circuit breaker. Combinations of screw sizes, pins, spacers and mating standoffs in the rating plug and/or circuit breaker may be employed to ensure that only certain rating plugs may be physically plugged into a circuit breaker.
Among other problems with such mechanical interlock schemes is that they are not conducive to flexibility or expansion. Groups of circuit breakers are limited by mechanical space and available screw sizes, lengths and pitches. Additionally, changes in a group of circuit breakers, such as a redesign that increases or decreases the frame size of a group of circuit breakers, would require a new mechanical interlock scheme that matches the appropriate values of rating plugs with the frame size of the group of circuit breakers. Designing new mechanical interlock schemes for each new iteration of circuit breaker families will quickly lead to increasing mechanical complexity in order to support further new circuit breaker designs. Thus, the use of mechanical interlock schemes is not conducive to expansion.
There is room for improvement in circuit breakers and associated methods.